The present invention relates to telephony and to telephone instruments which have dual circuit capability.
Most telephone networks in use at the present time are circuit switched networks, in which telephone subscribers have, at least to a local switching station, a dedicated line identified by a subscriber""s number. Connection to the network is achieved by a switch closing a circuit including the subscriber line to the local switching station. Telephone calls may be conveyed from switching station to switching station by a variety of links, but connection is established for the call to the ultimate recipient by means of circuit switching controlled by the generation of pulse trains or tones representing the recipient""s telephone number. Although telephone messages, whether voice calls or other types of call, such as facsimile transmissions, may be conveyed between switching stations by various time division, frequency division or even packet-switched communication links, as far as the end stations, the telephone instruments, are concerned they make connections to an analog circuit-switched network. Owing to the global nature of the existing circuit-switched networks, their use will obviously continue for many years and telephone instruments are necessarily adapted to establish connections by way of the familiar circuit-switched network.
In recent years there has been increasing use, first in local area networks and then on a wider scale, of packet-switched networks, of which the xe2x80x98Internetxe2x80x99 is the best known. The present invention is based on an appreciation that the usage of packet-switched networks will provide a potential alternative communication link between telephone subscribers.
The present invention envisages a new form of telephone instrument, typically a handset capable of making voice calls, which has a dual circuit capability in that it is capable of establishing a call connection over the ordinary, circuit-switched telephone network but may, if desired automatically, transfer the call connection from the circuit-switched telephone network to a packet-switched network.
An instrument according to the invention would normally possess, in common with existing instruments, some means for providing signals for transmission and some means for converting signals received by the instrument into output signals. The aforementioned means may, in the case of a telephone handset instrument, include the usual microphone and loudspeaker and the usual dialing equipment or tone generation equipment by means of which an intended recipient""s number can be transmitted in the format required by the circuit-switched network to the local switching station in order to establish a call connection with an intended recipient.
An instrument according to the invention will, however, further comprise not only a switching circuit for selectively establishing and terminating a connection between the instrument and the circuit-switched telephone network but further means for selectively establishing and terminating a connection between the instrument and a packet-switched communication network. The switching circuit for this purpose may include appropriate multiplexing equipment interposed between the call generating and receiving circuits and the terminals or connection points for the telephone network and the packet-switched network.
Further, the telephone instrument preferably comprises control means including means operable to define or obtain a network address of the instrument in the packet-switched communication network, the control means being operable:
(i) to control the switching circuit to establish a call connection to another instrument on the telephone network;
(ii) in response to receipt of an acknowledgement signal including the network address of the other instrument to control the switching circuit to make the call connection to the other instrument by way of the packet-switched network.
The instrument may signal information including its network address over the initial call connection. Alternatively, it may obtain its network address only after receipt of the said acknowledgment from the other instrument.
The instrument may further comprise control means operable on receipt of a call by way of the circuit-switched network to provide an acknowledgement including the network address of the instrument, and on receipt of a subsequent message over the packet-switched network to terminate connection over the circuit-switched network.